The invention relates to a chair mechanism for work chairs, particularly office swivel chairs, with synchronously adjustable inclination of the back rest and seat.
People employed in offices spend the major part of their working hours in a sitting position. It is taken as an established fact that the poor sitting position often adopted by them makes a considerable contribution to back injuries. A writing position, leaning forward, for hours at a time leads in particular to a considerable strain on the skeleton, as well as on the muscles and tissues of the back, and even to permanent curvature of the spine, the so-called hunchback (kyphosis).
This has given rise to many attempts to improve the geometry and the adaptability of chairs to different conditions of use with a view to healthier sitting. It has been found that frequent--even if only brief--relief of the human back by leaning back or leaning in a relaxed position already substantially reduces the strain on the back.
The construction of conventional office chairs, which apart from the swivellability was otherwise largely rigid, was scarcely suitable for effectively supporting the user's back. It is at best only in the upright position that such back rests in chairs of this kind have a positive action. A first improvement was made by Swiss Patent Specification 647 665, in which a back rest divided horizontally into two and comprising a fixed bottom part and a top part mounted resiliently thereabove was proposed. The construction enabled the user to lean backwards in a somewhat relaxed manner. An actual relaxing position was not however achievable, because, although the back rest partly adapted to the conditions of use at a given moment, nevertheless the seat surface did not harmoniously follow this adaptation but was mounted in a fixed position or could only be moved forwards and backwards.
Frame structures for office chairs were then further improved by enabling the seat to follow synchronously, by means of lever and joint connections, the adjustment of the back rest, so that ergonomically adapted positioning of the seat surface was associated with each inclination of the back rest. Synchronous mechanisms of this kind may be found in U.S. Pat. No. 4,773,703 or in U.S. Pat. No. 5,195,801; modifications are for example described in Swiss Patent Specification 629 945 and in German Offenlegungsschrift 37 35 256. These mechanisms enable the user to shift from the upright working posture into a backwardly inclined relaxing position, for example for a lengthy telephone conversation, by simply leaning the weight of the body backwards against the back rest, which usually has a damped spring action. In this movement the seat follows the inclining back rest in the same direction so that the user, after a lengthy upright or forwardly inclined sitting posture, assumes a position of rest which relieves his back and hip parts. With the aid of chairs having this type of configuration, which permit easy and frequent changes to a relaxing position for the user, it was possible to improve sitting comfort.
As a rule, however, office work is not done in an upright or relaxed backward leaning position, but in a posture ranging from a forwardly inclined to a hunched position in order to be in the closest possible contact, depending on eyesight, with the business papers lying on the desk. For the purposes of protracted desk work the chair constructions mentioned are of course only a partial improvement as regards a healthier sitting position, because the user can only very seldom adopt an upright or backwardly inclined posture, but on the contrary must predominantly retain a forwardly inclined sitting position. In this sitting position the front portion of the chair seat presses against the underside of the thighs, and the back rest does not effectively support the user's back part, while moreover the user also has no incentive to abandon the hunched posture.
After this had been realized, attempts were made also to alleviate in other ways the detrimental consequences of continuous sitting. Mechanisms were then proposed with the aid of which the seat surface was lowered in the forward direction (negative inclination) as the result of the shift of weight when the user adopted a forwardly inclined writing position. The user was thus induced to straighten his spine.
An apparatus of this kind is described in German Utility Model 81 33 573 for use as a typist's chair. A rocker is proposed therein which, through alternative installation of the operating lever from two different sides, is suitable in the one variant for a typist's chair which can be forwardly inclined from the normal vertical position, and in the other variant for an executive chair inclinable in the rearward direction from the normal position. This apparatus has the disadvantage that not all chair adjustments or movement cycles typical of an office can be achieved with it. In the one case only the normal position and the negative inclination, and in the other case the normal position and the relaxing position are possible.
Finally, an extreme form of chair with negative inclination and without a back rest came on the market. The user adopts a kneeling attitude in this chair, and the legs have to be bent backwards. His knees and shins rest on a support disposed at a height about halfway between the seat surface and the floor. Because of the obligatory squatting position, this construction is not very suitable for practical requirements in an office.
According to Swiss Patent Specification 650 136 another mechanism for an office chair having a lockable negative inclination is known. In order to give the user of this chair at least to a slight extent the ability to lean back, the back rest was divided horizontally into two and the top part of the back rest was resiliently mounted on the bottom part of the latter. The construction proposed here is very expensive, because two separate movement mechanisms--under the seat and in the back rest--are required. Furthermore, this chair also does not enable all sitting postures to be assumed or all movement cycles to be carried out.
There are thus now two directions for attempts to counter the harmful sitting posture in a continuous writing position--with the danger of curvature of the spine--by the constructional configuration of office chairs.
On the one hand the direction taken was the development of synchronous mechanisms to allow the user frequent short relaxation periods in a backwardly inclined relaxing position. At the same time it was attempted to prevent the backward rolling of the pelvis, resulting in a curved back, by means of a back rest configuration with lumbar support, often combined with the raising of the rear seat surface. The supporting action of the back rest is however attained only when the user is completely seated in the chair and makes firm contact with the back rest. In reality, however, most office workers do not sit completely in the chair, but sit further forward on the seat, and thus are given no support for their backs.
On the other hand, predominantly in Scandinavia, the direction taken was the development of mechanisms allowing a negative inclination for the writing posture of the user. The forwardly inclining seat surface is followed by the opposite reaction of the user's pelvis and thus by a forwardly inclined but nevertheless straight posture of the entire upper part of the body.
The generations of chairs produced to date provide only a partial solution to the problems of continuous sitting. Modern synchronous mechanisms are advantageous for users whose work permits frequent changes of posture. This relates, for example, to upright sitting during business discussions, a forwardly bent position for dealing with documents, and a relaxed posture during lengthy telephone conversations and intensive reflection. However, employees engaged mainly in desk work, that is to say in a forwardly inclined position, can make only little use of the advantages resulting from synchronous mechanisms. Such designs moreover are not ideal for rising from the chair. The user has to swing himself out of an unfavourable normal position--sitting completely in the chair in an upright attitude--with much effort and support by the arms. The other direction, in which the otherwise typical bad sitting position is corrected by a forwardly (negatively) inclined seat surface, soon leads to symptoms of fatigue through sitting on an inclined surface.
The springs used are of fundamental importance to the functioning of the mechanisms. To achieve the synchronous movement, widespread use is made of gas compression springs, which in some cases are also coupled to coil compression springs in order to optimize the movement cycle for the adjustment of the chair (see German Offenlegungsschrift 39 16 474, Swiss Patent Specification 629 945). The use of gas compression springs is however disadvantageous in many respects. Firstly, they are relatively expensive, thus affecting the cost of the entire chair. In addition, gas compression springs must be coupled to mechanical springs in order to achieve an advantageous movement cycle. This leads to further expense and the complication of the whole structure. Furthermore, because the seals are subject to wear, gas compression springs have only a limited useful life. A recent proposal was therefore to use a specially-shaped torsion spring (see International Publication WO 92/03072) for the spring system for the synchronous movement, as a mechanical solution free from problems.
Taking as starting point the previous short-comings of existing chair mechanisms and positive attempts to provide solutions, the problem underlying the invention is that of providing a mechanism which combines the advantageous tendency towards the straightening of the upper part of the human body through negative inclination of the seat surface, with the utilization possibilities offered by a synchronous mechanism. All sitting postures for the work to be done in an office should be achievable steplessly and continuously, in a manner imposing no strain on the hips and spine, with automatic adaptation solely through a change of posture or a shift of the weight of the upper part of the body, without support or lifting of the buttocks. The apparatus should be serviceable for the body weight range from about 45 kilograms to about 120 kilograms, and should offer special options, for example for persons whose body weights are very light or who have spinal problems. In addition, the apparatus must be produced with a compact construction in the interests of aesthetic configuration of the entire chair. Finally, the design and the components used must permit efficient and economical series production.